When power is plotted, as shown in FIG. 1, against current for a laser diode, unique characteristics are obtained for different ambient temperatures. Each characteristic has a linear portion which when extended would meet the current axis at a point known as the threshold current. When the laser transmitter is used in communications, the lasers are typically biased with a d.c. current near to but less than the threshold current. A modulation current is superimposed on the bias to swing the light output between a minimum and a maximum optical power corresponding respectively to logical zero and logical one. It is important that these minimum and maximum optical powers remain constant even when the laser threshold current and the slope of the characteristics change.
In the prior art, two different devices, known as single loop controllers and dual loop controllers, have been used to maintain the average optical power. According to one method, bias current is controlled. In this case the extinction ratio drops as ambient temperature rises. This problem occurs because of a corresponding decrease in the slope efficiency of its laser characteristic, that is, a decrease in efficiency with increasing temperature. To minimize this effect the temperature of the device was controlled.
According to the other method, both bias current and modulation current are controlled independently. This method determines peak and average optical output power via a backface monitor by measuring feedback current which is proportional to the light output power. This method requires high speed backface monitor and control circuits to measure the peak output power, hence the control loop must operate at the modulation rate.